The skinned muscle preparation, a single fiber in which the sarcolemma is either mechanically removed or chemically disrupted, is well suited for studying both tension and Ca regulating processes. Tension is regulated by the availability of substrate for the myosin ATPase and by the binding of Ca to troponin. The tensions generated by varying the substrate concentrations in the absence of Ca are described by a substrate-inhibition formulation. Ca modifies the relationship between tension and substrate by antagonizing the inhibitory effect of excess substrate. The applicability of this substrate-Ca regulatory scheme is currently being tested in a number of different skinned fiber preparations. These preparations are also useful for studying Ca regulations by the intact sarcoplasmic reticulum (SR) which remains functional after disruption or removal of the sarcolemma. The dependence of Ca regulation on the interaction amongst the Sr, troponin, and other Ca binding sites is being investigated.